Well, I’ve completed it! It took me around 30 hours, with a lot of aimless wandering around. Here’s a screenshot: (spoilers)

Final boss defeated
Here’s what my character’s stats looked like heading into the final room, definitely not how I would approach it on a new playthrough but it did the job. Endgame gear does most of the work tbh:

This game really captured my attention. I played the whole thing on the Steam Deck, which worked just fine. Performing magic runes on the little touchpad was surprisingly effective once I got the hang of it (I really think those touchpads are killer features on the Steam Deck, and a big reason why I have no interest in any of the competing handheld PCs on the market - especially for playing older games like this with unusual keyboard and mouse controls). The game was extremely obtuse in places, and I ended up using a lot of google searching to tie up all the loose ends.
There is so much detail everywhere you look here! The cooking system, hidden spells, hidden quests, all the secret passages and hidden item caches. I uncovered quite a lot of little things just by noticing something and messing around with it for a while - there is a real sense of joy uncovering secrets like that. I am sure there are many more as well. Information around the game is quite thin on the ground, and I had to get creative when trying to solve some problems, with satisfying results. For example (spoilers): there’s a bank in the main city which you can rob, but all the reading I did online indicated there is a certain part of the game where the bank manager is at home (which I missed), when you need to gain access to his house to complete the quest. Normally he is in this impenetrable cage at the bank, I was able to paralyze a guard with magic, then kill the bank manager through the wall with a fireball, then use telekenesis magic to loot the corpse while invisible to get his keys - all without alerting the guard in the same room!
I really get the sense that if I had access to this game when it released (in my early teens), I would have absolutely adored it, played through it multiple times and had a great time uncovering all its myriad secrets, talking on online messageboards about it, etc. I don’t have as much patience for that style of game experience nowadays, but I still had a great time nonetheless. I enjoyed the simple premise, the surprisingly diverse environments, the sense of humour the game has. Good stuff all round.
I haven’t played many Arkane games, I may have to check some more of them out! They have a new one out right? I think based purely on Arx Fatalis I can confidently purchase it forthwith, without wasting any time looking into reviews or online discourse and such.